1. Field of the Invention
The invention generally relates to Satellite Communication Systems and, more specifically, to equipment and method of measuring satellite linearity from earth using a low duty cycle pulsed microwave signal.
2. Description of the Prior Art
Linearity of any active device is determined by sending a Constant Wave (CW) signal to the device and measuring the resultant amplitude compression or amplitude compression and phase variation at the output of the device as the input power level is increased. Measuring the linearity of a satellite in orbit is a particularly difficult problem because the loss between the Earth station transmitter and the satellite is so extensive. Referring to FIG. 1, typical prior art testing equipment is generally designated by the reference numeral 10.
A CW signal generator 12 applies a CW signal to a CW high power saturated traveling wave tube amplifier (TWTA) 14 or a solid state power amplifier (SSPA). The output of TWTA 14 is sent to a satellite 16 by means of an Earth station antenna 18. The satellite 16 is preferably a “bent pipe” satellite. Most transponders are satellite communication channels that operate on a “bent pipe” principle. Such transponders send back signals that are received by a satellite with amplification, phase shift and frequency shift from the uplink to downlink frequency. The downlink signal is received by the Earth station antenna 20, the output of which is analyzed in a network analyzer 22. The TWTA 14 is used at the Earth station to transmit the signal at a sufficient power level to bring the satellite into saturation or near-saturation. Typically a high power amplifier, e.g. a traveling wave tube amplifier or a solid state power amplifier, is used at the Earth station to transmit the signal at a sufficient power level to bring the satellite into saturation or near saturation, as suggested in FIG. 1.
A system similar to the one described is disclosed in U.S. Pat. No. 4,038,600 to Thomas for a power control on satellite uplinks. This patent discloses a communications system using a linear transponder and the intermodulation products generated by the transponder to produce a control signal for controlling the output power of a transmitter such that the power level of the input signal to the transponder is maintained at a desired level. Intermodulation products are used to determine the power level of the input signal to the transponder. However, the system disclosed in this patent uses a test signal generator at the transmitter station that generates an unmodulated carrier in the form of a continuous CW signal. Because a continuous CW signal is used the average power transmitted is limited. This, in turn, limits the extent to which the satellite amplifiers can be saturated. As such, only a limited amount of information can be obtained about the transfer function of the satellite amplifiers based on the relatively low CW signals that can be transmitted in the uplink.
A series of related patents disclose receiver based saturation estimator circuits. In U.S. Pat. Nos. 7,054,391, 7,466,756, and 7,634,021 to Thesling the systems disclosed are for a method of estimating the saturation point of the transmitter from a transmitted communication signal received from the transmitter that generates a received data signal. The transmitted data signal is compared to the received data signal, and an estimate is made of the saturation level of the transmitter based on the comparison of the transmitted data signal to the received data signal. However, while these patents disclose a receiver-based saturation estimator the patents do not disclose the use of a pulsed microwave signal the duty cycle of which can be adjusted to selectively control and adjust the average power transmitted to the transponder to thereby modify the average power in the downlink amplifiers. Due to the inability to significantly modify the saturation levels of the downlink amplifiers limited information is available regarding the transfer function of those amplifiers and estimator circuits must be used to estimate the properties at saturation instead of accurately determining the same.